In microelectronics, interconnections between different parts of circuitry are often made with wires, or conductive material, having a diameter in the range of 0.001 to 0.002 inches and coated with a correspondingly thin enamel coating for insulation. The enamel coating is also beneficial in providing desirable mechanical strength to the thin wire.
Removal of the enamel coating along a specified length from the ends of each wire is necessary before forming the interconnections comprising the circuitry. The removal is difficult. The small diameter of the wire and the thinness of its enamel coating are physical obstacles to the efficient handling of the tiny wires while the insulation is removed. The difficulty in removing the insulation is compounded by the capacity of the enamel coating to strongly adhere to the wire, which minimizes the inadvertent removal of the coating.
Another significant factor contributing to the difficulty of removing the enamel coating is that the ablation must be sufficiently complete to provide a surface on the conductor that will reliably enable an electrical coupling with another conducting material.
It is known in the prior art to remove the insulation mechanically, with abrasives; chemically, with solvents; and electrically, with either an electric arc between electrodes or with a laser. There are objections to all of these known techniques. Abrasive methods are time consuming and present a high risk of mechanical damage to the conductors. The use of chemicals undesirably involves the use of toxic materials and the increasingly burdensome and expensive problem of disposing of toxic waste. The results obtained with either chemicals or abrasives are generally inferior to the results obtained electrically with either the prior art electric arc system or lasers. Ablation by laser is effective but generally economically unfeasible.
Ablation with an electrical arc presents less problems than the other known techniques for removing the dielectric coating from conductive materials. It is quick, inexpensive, and is capable of being used with little risk of damage to the conductive material. However, ablation with an electrical arc, as known in the prior art, lacks desirable efficiency because it frequently leaves an irregular residue of the insulation on the conductive wire, which prevents the reliable establishment of an effective electrical connection when installed in a circuit.
The industry has established a standard tolerance of 0.020 of an inch beyond the desired cut-off line, within which there may be residues of the coating or insulation. In other words, the industry recognizes that ablation of the end portions of a coated wire with the use of either an abrasive, a chemical, or an electrical arc inherently leaves a jagged or irregular line of cut or a spattered area of ablation with fragments of the dielectric coating adhering to the conductive material 0.020 of an inch, or further, from the desired cut-off line. The tolerance of up to plus or minus 0.020 of an inch is acceptable, but not desirable, because that is the closest tolerance the industry has been able to reliably achieve with the existing technology, other than some laser techniques.
U.S. Pat. No. 4,671,848, issued June 9, 1987 to General Laser, Inc. of Phoenix, Arizona for METHOD FOR LASER-INDUCED REMOVAL OF A SURFACE COATING tells the advantage of ablating by laser and describes the shortcoming in the use of an electric arc for ablation of a coating.
The laser induced removal of a dielectric coating on a fine gauge wire used in microelectronics is effective to make a clean cut without the inherent residue of the prior art electric arc system for ablation, but there is a high risk of damaging the underlying conductor with the typical laser process, and use of laser is generally awkward and effective only in low yield production.